Physical bodies exchange heat with their surroundings in order to achieve thermal equilibrium. Heat can be exchanged by conduction, convection, or radiation. A satellite in space is isolated from other materials and so cannot employ either conduction or convection for cooling. As a result, a satellite can be cooled only using radiative methods. One of the principles of radiative cooling is Kirchoff's Law, which states that in thermal equilibrium the processes of thermal absorption and thermal emission are equivalent. Said another way, things that absorb very well, e.g., a flat black panel, also emit very well.